Fluoroscopic C-arms are an important tool in computer-assisted interventions. For example, a fluoroscopic C-arm may be used in computer-assisted applications to reconstruct anatomical structures and objects in 3D from their 2D X-ray images. Reconstruction requires the relative pose of the 2D projection images, a problem commonly known as C-arm pose tracking.
Accurate, practical, and affordable C-arm pose tracking is a major technical challenge. There are motorized C-arm devices that provide rotational pose information, but they are rather expensive and susceptible to pose errors due to wheel motion, sagging, and deformation of the device. Conventional manual C-arms are supplanted with some tracking method to recover the
C-arm pose. Tracking may take two forms, external or X-ray image-based. In external tracking, an object called a dynamic reference body (DRB) is attached to the C-arm, while the pose of the DRB is sensed by optical cameras or electromagnetically [7]. External tracking produces full six degrees of freedom (DOF) pose of the C-arm and is resistant to wheel motion, sagging and deformation of the C-arm. Optical tracking is usually more accurate, but requires line of sight. Electromagnetic tracking does not demand line of sight, but is susceptible to field distortions typically caused by the presence of metallic objects or even the earth's magnetic field. A further shortcoming is that prior to using the system the DRB needs to be pre-calibrated to the C-arm image. In all, external tracking tends to add significant cost and complexity to the system. In image-based tracking, a radio-opaque object of known geometry (commonly referred as fiducial) is placed in the field of imaging [5, 11]. If the fiducial has sufficient specificity, then the full 6 DOF pose of the C-arm image can be computed relative to the fiducial. While image-based tracking is inexpensive and potentially accurate, the fiducial occupies valuable real estate in the image.
An alternative tracking method is the use of accelerometers to recover the pose. The initial theory postulated the use of accelerometers as a positional tracker, by performing a double integration of the acceleration information to retrieve positional information. Several previously published papers have used accelerometers in position tracking mainly for robotic applications [1, 8, 9]. However, applying similar methodologies to recover the C-arm pose proved problematic. Setting aside the inherent issues with accelerometers, such as noise, bias, and drift [10, 12], the mechanical properties of the C-arm did not allow for the accelerometer positional tracking methods to perform to an acceptable level of accuracy. The main issue stems from the C-arm rotations. The accelerometer senses motion and gravity forces. This means that when the C-arm rotates along its primary or secondary axis, the accelerometer will output readings caused by both forces. This makes it extremely difficult to discern the true motion required for pose tracking. If the accelerometer is coupled with a gyroscope, it will give orientation, which helps isolate the effects of static gravity. Knowing the static gravity, it can be subtracted from the accelerometer readings, thereby giving true motion acceleration at each time interval. This is a very complex and potentially costly strategy. Thus there is a need for a new technique to recover the C-arm fluoroscope rotation pose.